text: From August 1922 until the summer of 1923, Coster worked in Niels Bohr's Institute in Copenhagen. Within a few months he co-authored a landmark publication with Bohr, on X-ray spectroscopy and the periodic system of the elements. In addition he worked with chemist George de Hevesy on the identification of element No. 72.  Element 72 had been known to be a gap in the sequence of elements since 1914, when Henry Moseley created an experimental technique for placing the elements in a definite sequence. Radiochemist Fritz Paneth suggested that element 72 might be found in ores of zirconium. (Some histories incorrectly attribute this suggestion to physicist Niels Bohr.) Bohr published a prediction of the electronic configuration of element 72 in 1923. von Hevesy had been working with Bohr at the time.
  After Coster returned from Copenhagen he became Hendrik Lorentz' assistant at the Teylers Museum in Haarlem, where he developed an X-ray spectrometer. In 1924 he was appointed at the University of Groningen, where he was the successor of Wander de Haas. At Groningen he started an active research program in X-ray spectroscopy.
  In 1934 Coster became member of the Royal Netherlands Academy of Arts and Sciences.